Multiple electrode aggregometry will be performed using the impedance aggregometer and under the monitoring of an official representative of the manufacturer (Multiplate analyzer, Verum-Diagnostics, Munich, Germany). TPR results will be evaluated by 2 investigators who will be blinded to patient's identity and treatment. Whole blood will be sampled 24 hours after the loading dose. In patients who received IIb/IIIa inhibitor tirofiban blood samples will be obtained at least 24 hours after the completion of tirofiban infusion. On-treatment platelet reactivity (TPA) above 50%, compared to the basal value estimated by TRAP test, will be linked with low responsiveness.